On Wed, Mar 26, 2025 at 04:22:16AM -0700, Luis Chamberlain wrote:
> There are use cases where we wish to traverse the superblock list
> but also capture errors, and in which case we want to avoid having
> our callers issue a lock themselves since we can do the locking for
> the callers. Provide a iterate_supers_excl() which calls a function
> with the write lock held. If an error occurs we capture it and
> propagate it.
>
> Likewise there are use cases where we wish to traverse the superblock
> list but in reverse order. The new iterate_supers_reverse_excl() helpers
> does this but also also captures any errors encountered.
>
> Reviewed-by: Jan Kara <jack@xxxxxxx>
> Signed-off-by: Luis Chamberlain <mcgrof@xxxxxxxxxx>
> ---
> fs/super.c | 91 ++++++++++++++++++++++++++++++++++++++++++++++
> include/linux/fs.h | 2 +
> 2 files changed, 93 insertions(+)
>
> diff --git a/fs/super.c b/fs/super.c
> index 117bd1bfe09f..9995546cf159 100644
> --- a/fs/super.c
> +++ b/fs/super.c
> @@ -945,6 +945,97 @@ void iterate_supers(void (*f)(struct super_block *, void *), void *arg)
> spin_unlock(&sb_lock);
> }
>
> +/**
> + * iterate_supers_excl - exclusively call func for all active superblocks
> + * @f: function to call
> + * @arg: argument to pass to it
> + *
> + * Scans the superblock list and calls given function, passing it
> + * locked superblock and given argument. Returns 0 unless an error
> + * occurred on calling the function on any superblock.
> + */
> +int iterate_supers_excl(int (*f)(struct super_block *, void *), void *arg)
> +{
> + struct super_block *sb, *p = NULL;
> + int error = 0;
> +
> + spin_lock(&sb_lock);
> + list_for_each_entry(sb, &super_blocks, s_list) {
> + if (hlist_unhashed(&sb->s_instances))
> + continue;
> + sb->s_count++;
> + spin_unlock(&sb_lock);
> +
> + down_write(&sb->s_umount);
> + if (sb->s_root && (sb->s_flags & SB_BORN)) {
> + error = f(sb, arg);
> + if (error) {
> + up_write(&sb->s_umount);
> + spin_lock(&sb_lock);
> + __put_super(sb);
> + break;
> + }
> + }
> + up_write(&sb->s_umount);
This is wrong. Both the reverse and the regular iterator need to wait
for the superblock to be born or die:
void iterate_supers_excl(void (*f)(struct super_block *, void *), void *arg)
{
struct super_block *sb, *p = NULL;
spin_lock(&sb_lock);
list_for_each_entry{_reverse}(sb, &super_blocks, s_list) {
bool locked;
sb->s_count++;
spin_unlock(&sb_lock);
locked = super_lock(sb);
if (locked) {
if (sb->s_root)
f(sb, arg);
super_unlock(sb);
}
spin_lock(&sb_lock);
if (p)
__put_super(p);
p = sb;
}
if (p)
__put_super(p);
spin_unlock(&sb_lock);
}
> +
> + spin_lock(&sb_lock);
> + if (p)
> + __put_super(p);
> + p = sb;
> + }
> + if (p)
> + __put_super(p);
> + spin_unlock(&sb_lock);
> +
> + return error;
> +}
> +
> +/**
> + * iterate_supers_reverse_excl - exclusively calls func in reverse order
> + * @f: function to call
> + * @arg: argument to pass to it
> + *
> + * Scans the superblock list and calls given function, passing it
> + * locked superblock and given argument, in reverse order, and holding
> + * the s_umount write lock. Returns if an error occurred.
> + */
> +int iterate_supers_reverse_excl(int (*f)(struct super_block *, void *),
> + void *arg)
> +{
> + struct super_block *sb, *p = NULL;
> + int error = 0;
> +
> + spin_lock(&sb_lock);
> + list_for_each_entry_reverse(sb, &super_blocks, s_list) {
> + if (hlist_unhashed(&sb->s_instances))
> + continue;
> + sb->s_count++;
> + spin_unlock(&sb_lock);
> +
> + down_write(&sb->s_umount);
> + if (sb->s_root && (sb->s_flags & SB_BORN)) {
> + error = f(sb, arg);
> + if (error) {
> + up_write(&sb->s_umount);
> + spin_lock(&sb_lock);
> + __put_super(sb);
> + break;
> + }
> + }
> + up_write(&sb->s_umount);
> +
> + spin_lock(&sb_lock);
> + if (p)
> + __put_super(p);
> + p = sb;
> + }
> + if (p)
> + __put_super(p);
> + spin_unlock(&sb_lock);
> +
> + return error;
> +}
> +
> /**
> * iterate_supers_type - call function for superblocks of given type
> * @type: fs type
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index 1d9a9c557e1a..da17fd74961c 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -3538,6 +3538,8 @@ extern struct file_system_type *get_fs_type(const char *name);
> extern void drop_super(struct super_block *sb);
> extern void drop_super_exclusive(struct super_block *sb);
> extern void iterate_supers(void (*)(struct super_block *, void *), void *);
> +extern int iterate_supers_excl(int (*f)(struct super_block *, void *), void *arg);
> +extern int iterate_supers_reverse_excl(int (*)(struct super_block *, void *), void *);
> extern void iterate_supers_type(struct file_system_type *,
> void (*)(struct super_block *, void *), void *);
>
> --
> 2.47.2
>
